ˈsafety valve noun [countable]

1. a part of a machine that allows gas, steam etc to escape when the pressure becomes too great

2. something that allows you to get rid of strong feelings without doing any harm:
Being able to express emotion is a healthy safety valve for the relationship.

[TahlilGaran] Dictionary of Contemporary English ▲

a safety valve
a way of allowing someone to express strong or negative emotions without harming other people.
I often think football acts as a safety valve for a lot of stored-up male aggression.
(often + for)

[TahlilGaran] English Idioms Dictionary ▲